Seems to me you'd be better off with a circuit that up-converts 20 to 100 Hz sounds by a factor of 20, putting them into a range you can hear. That would not have to be a complicated circuit, and no doubt they're already out there.

Or it could convert sounds in that range into something that amplifies those frequencies and vibrates against your skin instead (a 'skin speaker') ... probably easier to make than a converter.